What type of jumps

what size

what type of surface dirt, gravel, or grass

Pardon me?

Maybe add a hill part?

maybe their aren't hills?...

In that case, get a dump truck and a lot of dirt :)

you have to have hills or some sort of vertical, end of story.

get a few flip up cardboard cartoon characters
install subterrainian monster jumps
paint everything black, trees, grass, dirt, etc...